Most internal and external grinding machines hold the workpiece in a rotating chuck. Grinding wheels or abrasive films held in shoes are then brought in contact with the inner or outer surfaces of the workpiece to smooth and finish these surfaces.
Centerless grinding refers to a broad class of metal removal techniques in which a generally cylindrical workpiece is both supported and rotated by two wheels or disks. In some configurations, one of the wheels is a regulating wheel which has an outer surface with a high coefficient of friction. The second disk is a grinding wheel which usually has a grit surface. The grinding wheel and the regulating wheel are rotated so that the surface velocity of the faces of each wheel is different. Since the surface of the regulating wheel has a high coefficient of friction, the workpiece tends to rotate with the regulating wheel but slide over the grinding wheel to the extent of the differences in surface velocities of the wheels. This sliding causes the slow gradual removal of metal along the outer circumference of the workpiece.
In other centerless grinding machines, a workpiece is held and rotated between two drive rollers which rotate with the same surface velocities. An abrasive element such as an abrasive film supported by a polishing shoe is then brought into contact with the outer surface of the workpiece to smooth the outer surface of the rotating workpiece. Still, other centerless grinding machines are adapted to remove material along an internal surface of the workpiece. These workpieces usually have a cylindrical inner bore into which a grinding head is introduced to smooth the surface of the bore.